1. Field of the Invention
The present invention relates to a semiconductor device for wireless communication. In particular, the present invention relates to a semiconductor device for wireless communication provided with a CPU (central processing unit).
2. Description of the Related Art
Recently a small-sized semiconductor device (a wireless tag) combining an ultra small IC chip and an antenna circuit for wireless communication has attracted a lot of attention. The wireless tag can read and write data by transmitting and receiving communication signals, with a wireless communication device (hereinafter referred to as a reader/writer). Although the wireless tag often has a card form or a chip form which is much smaller than the card form, it may have various forms depending on use.
As a field of application of the wireless tag, for example, product management in distribution industry is an example. These days product management with a barcode or the like is common, however, data cannot be read when there is an interrupting object in front of a barcode attached to the product because data stored in the barcode is optically read. On the other hand, in wireless communication which reads data wirelessly, data can be read as long as radio waves get through an interrupting object. Therefore, an efficient merchandise management and a cost cut in product management are expected. In addition, the wireless tag is expected to have a broad range of applications such as the railway ticket, the air ticket, and an automatic payment system.
The wireless tag including a memory is mentioned in the patent document 1 (Japanese Published Patent Application No. 2005-202947).
At present, the wireless tag employs, for example, a mask ROM (read only memory) as a memory for storing program data. The mask ROM is advantageous in a production cost, reading speed, and storing data. However, since data cannot be rewritten a plurality of times, data inside of the memory basically cannot be changed so the use of the wireless tag is limited once it is manufactured. In order to solve a problem with a limitation of the number of rewriting data, a flash memory may be used as a memory medium included in the wireless tag. In the case where the flash memory is provided to the wireless tag, although data can be electrically rewritten and stored, low reading speed is a problem. In addition, the flash memory takes longer for writing time than reading time and takes several hundred milliseconds for erasing time. Therefore, the wireless tag using the flash memory is inferior to existing wireless tags in response time. Moreover, the flash memory has a complicated process which leads to a high production cost so it is very difficult to provide the flash memory to the wireless tag which should be produced at a low cost.
In a conventional CPU system using a DRAM (dynamic random access memory) included in a PC (personal computer), the DRAM, a main memory, stores program data saved in a mask ROM when the CPU system is started. Features of the DRAM are high integration degree and a low production cost. However, operation of the memory is slow due to refreshing operation in which data is rewritten at regular intervals. A reason why refreshing operation is required is that the DRAM stores data by using accumulated electricity and the accumulated electricity is decreased by leakage current.